Thyroid hormone regulates expression of a transfected human alpha-myosin heavy-chain fusion gene in fetal rat heart cells.
The rat alpha-myosin heavy-chain (alpha-MHC) gene is regulated by 3,5,3'-triiodo-L-thyronine (T3) in ventricular myocardium and is constitutively expressed in atrial tissue. Less is known about regulation of the human gene, but conservation of sequences in the 5'-flanking region between the rat and human alpha-MHC genes suggests that the human gene may be regulated similarly. Accordingly, T3-responsiveness and tissue-specific expression of human and rat alpha-MHC/chloramphenicol acetyltransferase fusion constructs have been compared in rat fetal heart cells, L6E9 myoblasts and myotubes, 3T3 fibroblasts, and HeLa cells. Transient transfection assays revealed a complex series of cis-regulatory elements in the 5'-flanking sequences in the human genes, including a basal promoter element with canonical TATAA and CAAT sequences, two positive regulatory element(s), and two negative regulatory elements, which markedly diminished both constitutive and T3-inducible activity. Interestingly, the human gene seemed to contain a proximal thyroid-hormone response element(s) not found in the rat gene. In L6E9 myoblasts and myotubes, the human constructs were constitutively expressed but not T3-regulated; none of the constructs were active in 3T3 or HeLa cells. We propose that interactions among the thyroid hormone responsive elements and other cis-acting elements in the human alpha-MHC 5'-flanking sequences may be sufficient to explain the characteristic features of expression of this gene in cardiac tissues.
Tsika
R W
RW
University Heart Center, University of Arizona College of Medicine, Tucson 85724.
Bahl
J J
JJ
Leinwand
L A
LA
Morkin
E
E
D000818:Animals; D002478:Cells, Cultured; D015500:Chloramphenicol O-Acetyltransferase / Q000235:genetics; D002872:Chromosome Deletion; D005333:Fetus; D015870:Gene Expression / Q000187:drug effects*; D005796:Genes / Q000187:drug effects*; D006352:Heart Ventricles / Q000187:drug effects / Q000378:metabolism; D006801:Humans; D007700:Kinetics; D009206:Myocardium / Q000378:metabolism*; D015879:Myosin Subfragments / Q000096:biosynthesis / Q000235:genetics*; D010957:Plasmids; D011401:Promoter Regions, Genetic; D051381:Rats; D011993:Recombinant Fusion Proteins / Q000096:biosynthesis; D015183:Restriction Mapping; D014162:Transfection; D014284:Triiodothyronine / Q000494:pharmacology*
D015879:Myosin Subfragments; D011993:Recombinant Fusion Proteins; D014284:Triiodothyronine; D015500:Chloramphenicol O-Acetyltransferase